To date, the ocean has absorbed about 30% of the anthropogenic carbon dioxide (CO2) emitted since the industrial revolution, thereby causing declining pH and carbonate saturation states (Ω) throughout the surface ocean, i.e., ocean acidification (OA). Research into the impacts of ocean acidification has shown that lower saturation states cause calcification rates to decrease in many calcifying species. Saturation state monitoring is crucial to understanding the diverse impacts of ocean acidification on marine ecosystems and coastal economies in the next century.
The seawater CO2 system can be characterized by observations of the five measurable carbon system parameters: pH, total alkalinity (AT), total dissolved inorganic carbon (CT), CO2 fugacity (fCO2), and total carbonate ion concentration ([CO32−]T). Models of carbon system thermodynamics enable the calculation of all carbon system parameters (including calcite and aragonite saturation states) from any two of the measurable variables. For example, the parameter pair of CT and AT is used for saturation state calculations. However, measuring CT and AT requires two separate instrumental setups and protocols, and both protocols are somewhat complex and time-consuming. Additionally, attaining precise AT measurements requires meticulous gravimetric or volumetric measurements of the acid added during analytical titrations. This requirement can be especially challenging for shipboard measurements. A simpler, faster, and more convenient method for determining carbon system parameters, such as carbonate saturation state, would be beneficial for the widespread monitoring of ocean acidification.
Previously, indicators have been used to quantify titrant additions without the use of volumetric or gravimetric analysis. For example, seawater alkalinity measurements have been performed using a sulfonephthalein pH indicator dissolved in a hydrochloric (HCl) acid titrant to precisely quantify the concentration of acid that has been added using a spectrophotometer. Unfortunately, using such indicators requires meticulous preparation and characterization of the indicator-acid mixtures. In addition, such indicators can degrade or precipitate over time, thereby skewing results.
From the above discussion, it can be appreciated that it would be desirable to have a simple, fast, and convenient method for determining carbon system parameters of water, such as seawater, that does not require volumetric or gravimetric analysis and that does not require the addition of an indicator to an acid titrant.